Automatic volume control for keyboard operated musical instruments



P 1942- I J. L. TRAUB 2,296,125

AUTOMATIC VOLUME CONTROL FOR KEYBOARD OPERATED MUSICAL INSTRUMENT Filed May 6, 1941 3 Sheets-Sheet 1 sou-o GENERATOR mama? Ana PICKUP I I1 uentor A ttorlley our PUT To 1 mww rowan 1 P 1942- J. L. TRAUB 2,296,125

AUTOMATIC VOLUME CONTROL FOR KEYBOARD OPERATED MUSICAL INSTRUMENT Filed May 6, 1941 3 Sheets-Sheet 2 '7 5 i /g Inventor A Home Sept. 15, 1942. 1 L, TRAUB 2,296,125

AUTOMATIC VOLUME CONTROL FUR KEYBOARD OPERATED MUSICAL INSTRUMENT Filed May 6, 1941 I 3 Sheets-Sheet 3 Inventor Attorney Patented Sept. 15, 1942 AUTOMATIC VOLUME CONTROL FOB KEY- BOABg OPERATED MUSICAL INSTRU- dohn L 'lraub, Kenning, Minn.

Application May 0, 1941, Serial No. 392,148 Claims. (cl. ses'l) My invention relates to automatic volume controlling means for keyboard operated electrical musical instruments of the type employing an audio amplifier to amplify thesounds generated in the instrument when played, and the primary object of my invention is to provide means whereby the volume of the amplified soundsv may be controlled and modified in accordance with the wishes and playing ability ofv the player by varying the pressure on the keys and/or associated pedals of the keyboard. K

Another important object oi my invention is to provide a simple and eiiicient arrangement of the character indicated which may be embodied as an accessory. for easy application to an existing instrument or built-in as a unit in the original manufacture of the instrument at very little added expense.

Other important objects and advantages of my invention will be apparent from a reading of the following description and theappended drawin s wherein for purposes-of illustration 1 have shown a preferred embodiment of my invention.

In the drawings:

Figure l is a general schematic view showing one end of an instrument keyboard in accordance with the present invention and its mechanical connections with the electrical components, the electrical arrangements and connections of the last being shown diagrammatically.

Figure 2 is a contracted longitudinal vertical sectional view taken through the keyboard along the line 1-2 of Figure 1 and looking in the direction of the arrows.

Figure 3 is a contracted horizontal sectional view of the keyboard taken along the line 3-8 of Figure 1 and looking downwardly in the direction of the arrows.

Figure 4 is an enlarged transverse vertical sectional view taken through the variable resistor.

Figure 5 is a horizontal sectional view taken along the line ll of Figure 4 and looking in the direction of the arrows.

Figure 6 is an enlarged transverse vertical sectional view taken through the permanent magnet and coil arrangement.

Figure 7 is a wiring diagram showing within the dotted lines a circuit capable of being substituted for the main condenserto increase the effectiveness oi the suppressor grid.

Figure 8 is a fragmentary wiring diagram to illustrate factors of the variable resistance behavior.

When the player of a Hammond" electric organ, for instance, wishes to vary the volume of the sound output of the organ, he must manipulate a pedal with his foot, so that in play ing a dimcult passage on the pedal keyboard it is not possible for the player to vary the volume during such passage, and this is a serious drawback to artistic and correct rendition. Another serious fault of the present arrangement for varying the volume is that the pedal cannot be manipulated suiiiciently rapidly or accurately so as to enable, for instance, playing one note loudly and the next note relatively softly in a rapid tempo. i Because of such faults the scope of the mentioned and other similar instruments in their present forms is necessarily limited and control of expression and of volume are not at the disposition of the player as they are, for instance, in the piano. By the present invention I have succeeded in eliminating to a substantial extent these faults, so that the player can at will vary the volume of the manual. The mechanization of each of the eighty-eight notes of the manual is possible but impractical and too costly, but the arrangement provided by me aifords all that is necessary under ordinary circumstances for full musical rendition by th player through affording:

(1) More scope and variation of expression.

(2) Abilityto play the pedal keyboard without interruption to change the volume as now required.

(3) Much easier control of the volume.

(4) Capability of application to any number of manuals. I

Inasmuch as it is not intended to limit the invention to any particular instrument, only sufficient of such instruments is shown and described herein to disclose and explain the invention. Any of thesaid instruments has one or more manuals and some have pedal keyboards involving about thirty keys, all of these instruments fundamentally involving a stationary support I supporting the keys Ii for downward movement from the position shown in Figure l, a stationary key stop 8 surmounted by a felt 9 to limit the downward movement of the keys, a mechanism diagrammed at III which the keys t actuate to operate the electric organ or the like. and an upper stop ll provided with a felt I! to limit upward return movement of the keys.

Individual helical springs 2! are provided to yieldless engagement by the keys 6 as they are depressed by the player. An inverted L-shaped stop It is mounted on the foundation 5 in position to be engaged by a reduced felt equipped portion IQ of the key-bed bar Ii to limit the latters upward movement by the springs 20 which hold the floating key-bed bar up in position. As shown in Figures 1 and 2, the lever I is normally stationarily positioned horizontally. At different distances from its pivotal point the lever l5 has the horizontal adjustment slots 23 and 25, through which pass corresponding pivot bolts 26 and 21, secured to the upper ends of depending rods 23 and 29, respectively, whose functions are hereinafter described.

The floating key-bed bar It is a few inches longer than the distance between the lowest and highest keys of the manual, and i a light weight girder preferably of aluminum with the axle H consisting of steel stub shafts projecting from the opposite ends of the bar IS. The lever l5 likewise is preferably of aluminum so as to have minimum inertia.

The resistor R2 comprises the insulated superstructure 35 through which the rod or wire 29 works, the superstructure eiiectively closing the upper end of the mercury cup 3|. The latter has a concentric series of cylindrical walls 36 which surround the strip 30 and prevent splashing of the mercury 34 during rapid operation of the strip 30.

As shown in Figure 6 of the drawings, the magnet 33 may be of the same E-type as that of an ordinary loud speaker having its center element 31 wound at its base with the coil 38 the same as the field coil of the speaker. A guide bracket 40 for the rod or wire 28 is mounted on one side of the magnet whereby the hollow coil 32 is operable in correct relation to the leg 31 of the magnet 33.

The motion of lever is transmitted through the rod 29 to a resistance strip which is partly submerged in the mercury of the liquid container 3| that is part of R2. The motion of lever I5 is transmitted through rod 28 (which is physically but not electrically connected to lever IE) to the coil 32. The coil 32 is in the field of the electromagnet 33. The electro-magnet 33 and the coil 32 are connected in the radio tube circuit as shown in the diagram. Resistance R: is connected into the input circuit of the radio tube as shown in the diagram.

Operation Referring to Figure 2 of the drawings, when the key 6 is pressed down so that it just barely touches the top of key-bed bar IE it actuates the sound mechanism In so that an electrical pulse of this note is fed into the radio tube T1 at E1 with a certain arbitrary minimum of volume. If the key 6 is struck harder it pushes the floating key-bed bar l6 down an amount proportional to the pressure on the key and the lever l5 moves up causing the resistance strip of R2 to move up and also the coil 32 to move up. When the resistance strip 30 moves up this increases the amount of resistance in the circuit and the input voltage to the control grid of T1 is increased with a corresponding increase in volume of sound. The exact relationship will be given later. If the key is let up the volume will decrease until the point when the key ceases to touch bar H5 at which point the sound is disconnected from the amplifier. From the above it is evident that the volume of the notes played on a manual is dependent on the maximum finger pressure on the notes of the manual. The increase or decrease in sound is proportioned to the finger pressure by suitably proportioning R2. However, there is one fault with the system as above described. Examination will show that the volume will change during the time the key is moving. This would mar the music in some cases when the key is struck too slowly. This fault is remedied to a great extent by the coil 32 and magnet 33 and their connections to the amplifier circuit. The explanation is as follows: The lever [5 which is moved by the key 6 also moves the coil 32. The coil cuts the magnetic field of the magnet 33 and a voltage is generated in the coil 32. If the coil 32 moves upwardly the polarity of the voltage at the ends of the coil 32 will be of one sign. (The sign is dependent on the polarity of the pole of the electro-magnet which is adjacent to the coil.) If the coil moves downwardly the polarity at the ends of the coil 32 will be reversed. From this it is evident that when the coil moves in one direction a positive voltage will be applied to the suppressor grid G of tube T1. If the coil moves in the opposite direction a negative voltage will be applied to the grid G. What happens then is that when the lever l5 moves up an increase in volume due to R2 the coil generates a voltage of such a direction that a negative voltage is applied to grid G of T1. The negative voltage decreases the gain of the tube and the volume stays constant. However, as soon as the lever I5 stops then the coil stops and its generated voltage disappears and the volume increases to the point corresponding to the position of R2. The result then is the same if there is a decrease in pressure on the key. Then there will be a decrease in volume from the movement of the strip in R: but the grid G then becomes more positive and the amplifier gain increases so that the effect is that the volume remains constant or nearly so until the key stops moving. The circuit then avoids changes in volume during the time the key is moving. The bias battery for grid G is varied also for optimum operation. Selection of the right bias voltages will make grid G a linear control but this is not always desirable. The ear is the best judge in this matter of the selection of suppressor bias voltage. The output of T1 is fed into the power amplifier and the sound finally is heard from the loud speaker connected to the power amplifier. Consideration will show that when more than one key is pressed the heaviest pressed key determines the volume of the sound controlled by that manual, and letting a=the width of floating key bed bar l6 (same units as c).

0=the angle is radians which th bar l6 makes with a horizontal.

b=the distance (in some units as a) from the axis of the axle II to the point where rod 29 attaches to lever I5.

Then the displacement that resistor R1 experiences for C displacement of the key bed bar I6 is C (1)) units (same R2 (dlsplacement) cos 0 ]units as a, b and c) The discussion next will concern the minimum volum of R2, with reference to Figure 8 of the drawings.

Let

E1=pick up output voltage. R1=R1 in Figur 2. R2=R2 in Figure 2. Rr=control grid to cathode input resistance.

Then with the key or keys of the manual just touching the top of bar IS the resistor R (minimum) is adjusted to give a desirable minimum of volume (determined by ear) Since R1 is large in Figure it can be neglected in the following calculations. Let the resistance R (minimum) be denoted by Rm.

The next step is to decide on the maximum volume desirable which occurs when a key strikes the stop 8. This is determined by the requirements of the power amplifier and the power ofthe power amplifier is dependent on the maximum volume desirable for which the instrument is used. 'Let the volume equal 0 db. at the point where the key first strikes the bar l6 and causes a minimum volume. Then determine from the above the maximum gain and let it equal the volume H. For instance, H might be 30 db. in

which case a man with average hearing could detect distinct increased in volume.

Referring to Figure 4 the resistor R: is shown to be a thin insulating strip 30 coated with resistor material. The resistor makes contact with the circuit through the mercury 34 in the cup ii. Let d equal the displacement of the resistor strip 30 when the key 6 moves from minimum to maximum volume. Any other displacement would equal (from formula given above) constant Since n cos 0- let is equal q.

Then d'=c' (q) where 0 equals the distance the key bar It is depressed. From the above formulas it will be evident that the width of the resistor will vary with its length. Consideration of the formulas will show that the following relationship is true.

a (d/H)20 log which may be written m) 20d R2 d==c (q) :displacement q lever arms I! (linear units.

H=maximum gain in db.

d=maximum displacement q lever arms -15.

Rm=R (minimum) see Fig. 2.

R1==R1 Figure 2.

Rz=variable resistor R2 Figure 2.

Equation 4 shows how R: should be designed. If the displacement c of the key bed bar is used instead d' (the displacement of the lever l5 at 21) then substitute 0' (q) for d in Equation 4. I ,will now take up the discussion of the coil 32 and magnet 33. It is desirable that wire or rod 28 be placed near to axle I4 on lever 15 so that the coil 32 will move in a uniform field of the electromagnet 33. By means of variable resistor R: and movable U on R5 the circuit can be adjusted to correct conditions. It will be seen that this is a matter of cut and try. The circuit must be .adjusted by means of Ra, contact U and the position of 28 on II until the correct voltage is generated to control the tube correctly.

I now take up the discussion of the bias voltages on T1. Grid bias resistor Re is adjusted so that when the suppressor grid is grounded the tube acts like an ordinary class A amplifier. The bias battery W connected to the suppressor grid linearly controls the output of Ti. The battery should put the tube on the middle of the linear slope. This can be done by means of an oscilloscope or output meter. Resistor R1 is chosen so that it is not too small so that the grid is not dependent only on Run and not too large so that R: has to become impractically large to cause the required variation in volume.

I should point out at this time that condenser C1 is one of the most important parts of my invention. Its value can best be determined by trying difierent values and selecting the value which sounds best to the ear. The function oi the condenser is to smooth out the voltage pulse from the coil 32 so that the suppressor more effectively maintains the volume constant during the time a key or keys are changing position. In this connection in order to increase the effectiveness of the suppressor grid the condenser C1 may be replaced by the circuit shown in Figure 7.

The circuit replacing Cl is enclosed in the dotted lines. Rnl0,000 ohms. The symbol a means approximately. Condensers Cu and Cm are chosen by trial. It must be emphasized that with this circuit the bias battery W must always put a negative voltage on the grid G regardless of how great the voltage due to the coil becomes.

Summarizing then, when Ei is given and the audio power output is given then all the constants of the circuit are determined except C1.

B1, Rm, R2, coil and magnet. Rm is determined by the minimum volume. R: and R1 is determined by the maximum volume. The coil and magnet are determined so that sufficient voltage is generated by the coil to effectively control grid G. Ci is determined by ear. Briefly reviewing the operation, the coil counteracts the resistor R: during the time the bar It is moving.

The condensers shown are of the following capacities:

C4=0.1 mfd. Cz-2.0 mfd. Ca=2.0 mfd. C5 49 mfd.

C1=variable as discussed above.

The capacities of the resistors are as follows:

R5 (voltage divider) =50,000 ohms. Ra=2000 ohms (with potentiometer). R4=250,000 ohms.

Ri==variable as discussed above. Rz=variab1e as discussed above. Ra=3500 ohms 0.1 watts.

R1=50,000 ohms 0.1 watts.

The tube T is a type 5'7 radio tube. W is the suppressor grid bias battery.

Although I have shown and described herein a preferred embodiment of my invention it is to be understood that I do not limit the invention thereto except as may be required by the subjoined claims.

Having described the invention, claimed as new is:

1. A device for electrically reproducing and amplifying the sound of a keyboard musical instrument caused by depression of one of the keys thereof in accordance with the amount that the key is depressed, said device comprising an electrical pick-up and an audio amplifier operated thereby, a variable resistor connected in circuit between the output of said pick-up and said amplifier to vary the input to the amplifier, said resistor comprising an element movable to vary the capacity of the resistor, and mechanical means operatively connecting said key with said movable element, said amplifier comprising an electronic amplifier tube having a suppressor grid, and compensating means comprising a connection with said movable member, said compensating means being effective upon movement of said movable member in one direction to impress a positive voltage on said suppressor grid and in the opposite direction to impress a negative voltage on said suppressor grid, to equalize the volume output of said amplifier during variations in capacity of said variable resistor due to the movement of said key toward and away from its maximum depressed position.

2. A device for electrically reproducing the sound emitted by a keyboard musical instrument caused by depression of one of the keys and amplifying the sound in accordance with the amount that the key is depressed, said device comprising an electrical pick-up and an audio amplifier operated thereby, a variable resistor connected in circuit between the output of said pick-up and said amplifier to vary the input to the amplifier, said variable resistor comprising a relatively stationary element and a movable element mechanically connected with the key, said movable element being arranged for movement relative to what is said stationary element in accordance with the amount the key is depressed, a variable impedance comprising a relatively stationary member and a movable member, said movable member being mechanically connected to said key, said movable member being arranged to move relative to said stationary member in accordance with the amount that the key is depressed, said amplifier comprising an electronic amplifier tube having a suppressor grid, said impedance being electrically connected to the eflective to impress a positive voltage on said grip when the movable member is moved in one direction and to impress a negative voltage on said grid when said movable member is moved in the opposite direction and while corresponding movement of the movable element of said variable resistor in takin place, whereby to positively equalize the volume output of the amplifier during the transit of the key toward and away from. its maximum depressed position.

3. A device according to claim 2 wherein said stationary element of the variable resistor comprises a container having a conductive liquid therein, and wherein said movable element comprises a tapered electrode adapted to become immersed in the liquid an amount corresponding to the amount the key is depressed.

4. A device according to claim 2 wherein said stationary member of the impedance comprises a permanent magnet core equipped coil and said movable member comprises hollow coil circumposed upon and movable relative to the permanent magnet coil, said hollow coil being adapted to telescope onto said permanent magnet coil an amount corresponding to the amount the key is depressed.

5. A device according to claim 2 whereinthe movements of the movable element of the resistor and of the movable member of the impedance are mechanically coordinated and proportioned with respect to the movements of the key.

JOHN L. TRAUB. 

